1. Technical Field of the Invention
The present invention relates generally to an improvement on a gas sensor which may be employed in an oxygen measuring device of an air-fuel ratio control system measuring an oxygen content in exhaust gasses of an internal combustion engine of automotive vehicles.
2. Background Art
For burning control of fuel in internal combustion engines, modem automotive vehicles use a gas sensor, e.g., as an oxygen sensor which is installed in an exhaust system to measure the concentration of oxygen in exhaust gasses.
European Patent Application EP 0918215 A2 teaches an oxygen sensor designed to define an air gap between an insulation porcelain and a metallic cover which is large enough for admitting air used as a reference gas in determining the concentration of oxygen. FIG. 21(a) illustrates the insulation porcelain disclosed in this application. The insulation porcelain 9 consists of a large-diameter portion 92 and a small-diameter portion 91. The small-diameter portion 91 is of a rectangular shape and has formed therein through holes 30 within which lead lines are held. The insulation porcelain 9 is fitted within a metallic cover (not shown) to define the air gap between an inner wall of the metallic cover and the small-diameter portion 91.
The formation of the insulation porcelain 9, however, experiences, as shown in FIG. 21(b), the deformation of the small-diameter portion 91 in compressing the ceramic powder because the interval O between an outer wall 911 of the small-diameter portion 91 and an outer wall 921 of the large-diameter portion 92 varies in a circumferential direction of the insulation porcelain 9, thus resulting in a decreased strength of the insulation porcelain 9. This problem is common to gas sensors of the type having a reference gas chamber admitting a reference gas used in determining the concentration of a specific gas.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an improved structure of a gas sensor capable of admitting a sufficient amount of a reference gas into a reference chamber without scarifying the strength of an insulation porcelain.
According to one aspect of the invention, there is provided an improved structure of a gas sensor designed to measure a given component content in a gas. The gas sensor comprises: (a) a housing; (b) a sensing unit having a length disposed in the housing, the sensing unit having defined in a first end portion thereof a reference gas chamber to be filed with a reference gas used in providing a sensor signal through a lead which is employed in determining the given gas component content in the gas; (c) a first metallic cover installed on the housing to cover a second end portion of the sensing unit; (d) a second metallic cover installed on a periphery of the first metallic cover; (e) a first vent formed in the first metallic cover; (f) a second vent formed in the second metallic cover which communicates with the firs vent to admit the reference gas into the reference gas chamber through a reference gas passage; and (g) an insulating member disposed in the first metallic cover, having formed therein a hole through which the lead passes to connect with the sensing unit, the insulating member being made of a cylindrical porcelain having an outer peripheral wall which is substantially circular in cross section and which defines the reference gas passage.
In the preferred mode of the invention, the insulating member has a first end surface and a second end surface opposed to the first end surface in a longitudinal direction of the gas sensor parallel to the length of the sensing unit. The insulating member has a through hole extending in a direction of the first end surface to the second end surface to define a portion of the reference gas passage.
The insulating member is arranged in alignment with the sensor unit and has a groove formed in the outer peripheral wall which extends from the first vent to the first end surface to define a portion of the reference gas passage.
The insulating member has a small-diameter portion formed closer to the first end surface and a large-diameter portion continuing from the small-diameter portion. If a length of the small-diameter portion in a direction is defined as L1, a distance L2 between the large-diameter portion and an upstream end of the groove facing the first vent lies within a range of L1/5 to L1/2.
The first vent has a diameter R in the longitudinal direction of the gas sensor. The distance between a point on a periphery of the first vent closest to the second end surface of the insulating member and an upstream end of the groove facing the first vent is greater than or equal to R/3.
The insulating member may alternatively have a groove formed in the outer peripheral wall which extends from the first vent to the second end surface to define a portion of the reference gas passage.
If a plane tangent to a periphery of the insulating member is defines as P, a plane passing through the deepest point of the groove in parallel to the plane P is defined as P1, and a plane passing in parallel to the plane P through the center of the through hole formed in the insulating member is defined as P2, a distance S1 between the planes P and P1 is smaller than or equal to a distance S2 between the planes P and P2.
If a width of the reference gas passages defined on the outer peripheral wall of the insulating member is defined as H1, and a diameter of the insulating member is defined as H2, they are so selected as to meet a condition of H1xe2x89xa6H2/21/2.
The insulating member may alternatively have formed therein a plurality of lead holes through which leads pass to connect with the sensing unit. The reference gas passage may be defined at a location where a line passing through a center of the insulating member between adjacent two of the lead holes intersects the outer peripheral wall of the insulating member.
The reference gas passage may alternatively be defined by a hole formed in the insulating member which extends from a portion of the outer peripheral wall of the insulating member facing the first vent and communicates with the hole through which the lead passes.
The insulating member may have formed therein a lateral hole extending between the lead holes in communication with the through hole extending in the direction of the first end surface to the second end surface of the insulating member to define the reference gas passage.
The reference gas passage may alternatively be defined by a through hole formed in the insulating member which extends from a portion of the outer peripheral wall facing the first vent to the chamber through the small-diameter portion and the large-diameter portion.
The reference gas passage may alternatively be defined by an inner wall of the first metallic cover and a surface of the outer peripheral wall of the insulating member tapering off to the first end surface.
The reference gas passage may alternatively be defined by an inner wall of the first metallic cover and a first and a second annular step formed on the outer peripheral wall of the insulating member. The first annular step is smaller in diameter than the second step.